The present invention primarily relates to electronic security systems, and in particular, to an improved antenna structure for an electronic article surveillance system.
A variety of electronic article surveillance systems have been proposed and implemented to restrict the unauthorized removal of articles from a particular premises. One common form of this is the electronic article surveillance system which has come to be placed near the exits of retail establishments, libraries and the like. However, electronic article surveillance systems are also used for purposes of process and inventory controls, to track articles as they pass through a particular system, among other applications.
Irrespective of the application involved, such electronic article surveillance systems generally operate upon a common principle. Articles to be monitored are provided with tags (of various different types) which contain a circuit (a resonant circuit) for reacting with an applied radio-frequency field. A transmitter and a transmitting antenna are provided to develop this applied field, and a receiver and a receiving antenna are provided to detect disturbances in the applied field. If the active circuit of a tag is passed between the transmitting and receiving antennas (which are generally placed near the point of exit from a given premises), the applied field is affected in such fashion that a detectable event is produced within the receiver. This is then used to produce an appropriate alarm. Systems of this general type are available from manufacturers such as Checkpoint Systems, Inc., of Thorofare, N.J., among others.
Although such systems have proven effective in both security as well as inventory and process management, it has been found that certain enhancements to such systems would be desirable. Perhaps foremost is the ever-present desire to reduce to the extent possible any errors (e.g., false alarms) which are produced by such systems, particularly in terms of their discrimination between the presence of a tag (signifying the presence of a protected article) and other interference which may be present in the vicinity of the electronic article surveillance system. Any steps which can be taken to enhance the accuracy of the system will tend to reduce such undesirable results. However, complicating this is the corresponding and at times conflicting requirement that any fields which are produced by the system must meet prevailing FCC regulations, limiting the nature (frequency, intensity, etc.) of the fields which may be used in making such determinations.
One factor which contributed to this relates to the antenna structures which are used in conjunction with the system's transmitter and receiver, in order to develop and pick up the particular radio-frequency signals which are utilized in such systems. In particular, the known antenna structures had response characteristics, within the range of radio frequencies typically employed, which could introduce amplitude and/or phase distortions into these signals which were sufficiently pronounced to contribute to false alarms.